With steam reformer furnaces for generating hydrogen in oil refineries, a gas mixture of a petroleum fuel (naphtha, crude gasoline or the like) and steam is fed to a reaction tube and subjected to a reforming reaction [CmHn+mH2O→(n/2+m)H2+mCO] at a high temperature and high pressure (temperature: about 700-900° C., pressure: about 1 to 3 MPa) in the presence of a catalyst to produce a synthesis gas consisting mainly of hydrogen. The reforming reaction tube needs to have a high-temperature strength and high-temperature creep strength so as to withstand a continuous long-term operation under the conditions of high temperature and high pressure. Heat-resistant high-carbon, high-Cr—Ni cast steel, which is a precipitation hardening alloy, has conventionally been used as a material for making the reaction tube. Stated more specifically, the materials heretofore developed up to date are SCH22(0.4C-25Cr-20Ni—Fe) as the first-generation material, IN519(0.3C-24Cr-24Ni-1.5Nb—Fe) as the subsequent second-generation material, and micro alloying materials, such as HP-Nb, Ti alloy (0.5C-25Cr-35Ni—Nb, Ti—Fe) containing a very small amount of Nb, Ti or the like, as the third-generation materials.
Patent Literature 1: publication of JP-B No. 55-47105
Patent Literature 2: publication of JP-B No. 57-40900
Patent Literature 3: publication of JP-A No. 5-239599
In recent years, there is a growing demand for clean energy as a measure against environmental pollution, and fuel cells for use with hydrogen fuel have attracted attention and are thought promising, for example, for use as power sources for motor vehicles. Fuel cells are developed also as distributed power sources on a small scale and partly introduced into actual use. Along with this trend, great efforts are made for developing hydrogen generation systems for supplying hydrogen to fuel cells. These systems include small hydrogen generators and on-site hydrogen generation units (such as so-called “hydrogen stations”) for producing hydrogen from liquefied petroleum gas (LPG), town gas (LNG) and hydrocarbons including alcohols, kerosene and light oil.
The steam reforming reaction of hydrogen generation systems for fuel cells is conducted at temperature and pressure (temperature: about 750-800° C., pressure: up to about 1 MPa) relatively lower than the operating conditions for large-sized apparatus in oil refineries, whereas the fuel cell power demand greatly fluctuates between the daytime and the nighttime, with the result that the operation of hydrogen generation systems involves repeated fluctuations in the load on the reforming reaction tube in corresponding relation with the power demand. If such load fluctuations are repeated daily, creep and fatigue accumulate on the reaction tube, leading to fatigue failure. In addition to high-temperature strength and high-temperature creep rupture strength, outstanding fatigue characteristics are therefore required of the reforming reaction tube for use in fuel cell hydrogen generation systems. Although the heat-resistant high-carbon, high-Cr—Ni cast steel of the precipitation hardening type for use in the large-sized apparatus of oil refineries has high-temperature characteristics (high-temperature strength and creep rupture strength) required for a continuous operation at a high temperature and high pressure, the steel still remains to be improved in aged ductility and resistance to fatigue failure required of hydrogen generation systems of the load fluctuation type and is not usable with good stability over a prolonged period of time. When used in an environment having a temperature range of about 800° C. for a long period, the steel also encounters the problem of embitterment resulting from the precipitation of sigma phase and pointed out in the case of the material HK40.